Several studies are designed to examine the role of pineal melatonin in mammalian photoperiodism. The experiments are largely based on our observations in Djungarian hamster which demonstrated that (1) the duration of the nocturnal melatonin peak varies as a function of the photoperiod and (2) long duration melatonin peaks elicit responses which are typically exhibited during exposure to short days, while short duration peaks trigger long day type responses. Attempts will be made to answer the following questions: Is a circadian clock involved in the melatonin response system? Does melatonin act at multiple sites to exert its several physiological effects? How does the mother hamster transmit photoperiodic information to her fetuses? Does the pineal melatonin rhythm exhibited by rodents under field conditions conform to that described for laboratory housed animals?